Formation test tools conventionally collect fluid samples in a sample chamber which has a known volumetric capacity, although the volume of the actual fluid sample taken downhole may not be known. A sample tool typically includes a cylinder fitted with an end cap and a separator piston for sealing engagement with the inside diameter of the cylinder. The volume within the cylinder between the end cap and the separator piston determines the volume occupied by the fluid sample. Due to various types of buffering systems used in formation test tools and the cooling effects on the fluid sample as it is retrieved to the surface, the volume of the collected sample at surface temperature and pressure is unknown.
One type of sample chamber arrangement utilizes an air cushion to buffer the formation fluid flowing into the chamber. A flow line conventionally connects a packer associated with the formation test tool to the inlet of the air cushioned sample chamber. Prior to sampling, the separator piston is in an uppermost position adjacent the end cap, and the volume below the separator piston is filled with air, usually at atmospheric pressure. When a sample formation fluid is to be collected, a flow line control valve is opened to fluidly connect the flow line to the inlet of the sample chamber. Formation pressure is substantially higher than air pressure below the separator piston, such that the formation fluid pushes the separator piston downward, thereby filling the chamber with formation fluid. The separator piston will continue to move downward until pressure across the separator piston is substantially equalized. The flow line control valve may then be closed, trapping the collected sample within the sample chamber.
Another formation test tool utilizes a liquid cushion as a buffer to control the rate at which formation fluid would fill the chamber. The buffer fluid below the separator piston is a relatively incompressible fluid, such as water or ethylene glycol. When the control valve is open, pressurized formation fluid acts on the separator piston forcing the buffer liquid through a restriction or choke. The region below the choke is initially filled with air at atmospheric pressure. Collected fluid pressure thus forces the buffer liquid through the choke and into the air filled choke chamber, thereby compressing the air. The separator piston will continue to move down until the pressure of the compressed air is approximately equal to the formation pressure, at which time the control valve may be closed to trap the collected sample within the cylinder.
Yet another technique for collecting downhole fluid samples utilizes a downhole pump as part of the formation test tool. The pump is connected to the flow line of the tool which is in fluid communication with the downhole formation of interest. The outlet of the pump is directed to the inlet of the sample chamber. The lower side of a separator piston is exposed to wellbore fluid at hydrostatic pressure. As the chamber is filled, the pressure of the fluid sample is increased by the pump from approximately formation pressure to a value equal to or higher than hydrostatic pressure. As formation fluid is pumped into the chamber, the separator piston moves downward, displacing the well fluid below the piston. Downward motion of the piston continues until the piston reaches its full extent of travel. The possibility of a flow line plugging during the sample filling procedure reduces the probability of a desired volumetric sample in the chamber.
Yet another formation test tool utilizes nitrogen or another compressible gas pressurized to downhole conditions to compensate for sample contraction upon cooling when the sample is retrieved to the surface. A nitrogen compensated sample chamber is separated by a piston adjacent the upper end cap when formation fluid is initially input to the cylinder. Nitrogen gas is contained between the separator piston and a lower charging piston. Wellbore fluid at hydrostatic pressure is exposed to the lower side of the charging piston. The outlet of the downhole pump is directed to the inlet of the sample chamber. As the chamber is filled, the separator piston, the nitrogen gas and the charging piston are pushed downward until the charging piston reaches its full extent of travel. Additional pumping displaces the separator piston, compressing the nitrogen charge. Once over pressurized to a desired value, the flow line control valve is closed to trap the collected sample.
U.S. Pat. No. 3,530,933 discloses a formation sampling tool for sampling downhole fluids. The tool includes an annular sealing pad for engagement with the wellbore surface. The flow of the fluid sample may be regulated for limiting the entrance of mudcake and formation materials that may plug the sampling system.
U.S. Pat. No. 3,577,783 discloses a downhole sampling tool with two pistons of a differential diameter within a cylinder. A delay is provided by a choke in a branch conduit or by a check valve and piston cylinder assembly in the branch conduit.
U.S. Pat. No. 3,677,080 discloses a formation sampling tool run on a logging cable. Pressure balancing pistons prevent hydrostatic pressures from interfering with tool mechanical functions.
U.S. Pat. No. 3,952,588 discloses a tool with a movable chamber expanded to draw mudcake and plugging materials into a receiving chamber. The chamber is shifted to thereafter communicate a screened entry port with the isolated formation.
U.S. Pat. No. 4,339,948 discloses a well formation test apparatus with a sealing pad arrangement to seal the test region and permit the flow of formation fluid from the region. A pressure detector senses and indicates a buildup of pressure from the fluid sample.
U.S. Pat. No. 4,879,900 discloses a formation tester with multiple sample storage containers and an equalizing valve to selectively isolate a snorkel from the fluid and pressures in the well. A control valve system operates backup shoes and a snorkel seal until testing is complete.
U.S. Pat. No. 4,936,139 discloses a formation test tool which utilizes a straddle packer to allow formation specimens to be taken at large flow rates.
A significant disadvantage of prior art formation test tools is that the operator at the surface cannot reliably verify the volume of fluid within the test tool. When the sample chamber is retrieved to the surface, the sample fluid cools and shrinks to reduce the sample pressure. This reduction in pressure will reposition the separator piston until a piston equilibrium pressure is established on both sides of the piston. Not being able to determine the volume of the downhole collected sample has significant drawbacks, since the sample cylinder may be shipped at considered expense to a laboratory for analysis only to determine that fluid has not been properly collected due to a leak in the system or due to a malfunction of the equipment. The operator at the surface may use a pressure gauge to determine the pressure of the sample fluid at the surface, but the inability to determine the volume of the sample fluid does not allow a meaningful correlation between known or presumed downhole temperature and pressure conditions, and the affect of the cooler surface temperature and lower pressure conditions on the sampled fluid.
The disadvantages of prior art are overcome by the present invention. Improved equipment and techniques for determining the volume at the surface of a transportable sample cylinder which houses downhole fluids may be made before shipment to a laboratory for sample analysis.